Ideally, no current flows into the input terminal of an amplifier. In practice, however, there are two input bias currents, IB+, IB−. These currents times the external impedance associated with the signal driving the input causes a voltage which is an error voltage between the actual signal and what the amplifier sees.
Compensating for the input bias current reduces this error voltage, but typically increases the standing current, increases the input current noise, and limits the bandwidth of the amplifier. For example, compensating for the input bias current by supplying a current equal to half the tail current of the amplifier's differential pair through a transistor that is similar to the input transistors and mirroring the base current of this secondary transistor back to the input transistors' bases increases the standing current and the input current noise. Another method buffers the collector current of the differential input transistors with cascode transistors and mirrors the base currents of the cascode transistors back to the input of the amplifier, but limits the bandwidth and positive common mode range of the amplifier because the cascode transistors are in the signal path.